Crystal rectifiers and method



P 1958 'J. J. DYMON ErAL 2,852,448

CRYSTAL RECTIFIERS AND METHOD Filed Sept. 1, 19155 INVENTOR. 0 55ENH/LQD58/422215 1 By 050% J fim/a/r NJHW United States CRYSTAL RECTIFIERS ANDMETHOD Application September 1, 1955, Serial No. 532,006

4 Claims. (CL 204-29) The present invention relates to the manufactureof electric current rectifying devices, and more particularly deals withthe processing of rectifiers utilizing a single crystal of strontiumtitanate as the rectifier body, and to such rectifiers as improvedarticles of manufacture.

Structurally, a crystal rectifier includes a body or layer ofsemiconductive material to which are attached two metal contacts servingas electrodes. One of the contacts is essentially ohmic and isdesignated as the base electrode, while the other of the contacts allowsfor heavy current flow in the forward direction and has a high reverseresistance, and serves as a rectifying junction or barrier. Numerousmaterials are known to be suitable for the semiconducting layer,including germanium, silicon, sintered bodies of titanium dioxide, andsintered bodies of alkaline earth titanates and mixtures thereof. For acomplete description of rectifiers incorporating a semiconductive bodyprimarily of an alkaline earth titanate in a reduced state, referencemay be made to copending applications Serial No. 341,726 filed March 11,1953, now Patent No. 2,820,184, Serial No. 365,987 filed July 3, 1953,now Patent No. 2,806,189, Serial No. 366,038 filed July 3, 1953, SerialNo. 399,456 filed December 21, 1953, now Patent No. 2,796,564, all ofwhich applications are assigned to the assignee of the presentinvention.

' Broadly it is an object of the present invention to provide improvedrectifying devices of the aforesaid type. Specifically it is within thecontemplation of the present invention to provide useful rectifiers,both of the point contact and area contact types, incorporating reducedsingle crystals of strontium titanate.

As detailed in the aforesaid application, ceramic bodies of reducedtitanates of the alkaline-earth metals, particularly those titanates ofbarium, strontium, calcium and mixtures of barium, strontium and calciumare suitable for incorporation into rectifiers after appropriatereduction, surface treatment and the application of electrodes andcounter-electrodes. Such ceramic semiconductive bodies include a largenumber of small crystalline particles or grains which are compacted andsintered together and may be characterized as highly polycrystalline.The orientation of the crystallographic axes of these particles orgrains are random; and between particles or grains there exist grainboundaries Within which occurs the transition from the crystal orlattice orientation one grain to the crystal or lattice orientation ofthe next adjacent grain. As a result of the random orientation of thecrystallographic axes, it is impossible for the material within thegrain boundaries to show the high degree of atomic order associated witha crystalline material; rather such sintered bodies may be expected toshow a high degree of disorder on an atomic scale. The occurrence ofgrain boundaries in the rectifying barrier or surface presents regionsof weakness, that is defective spots or localized areas of lowresistance which are likely to cause rectifier failure.

atent In accordance with the present invention, it has been 2 found thatsingle crystals of strontium titanate are useful as the semiconductingmaterial in rectifying devices, both for point contact rectifiers whichare exceptionally useful for communications applications in the highfrequency and microwave frequency ranges and for area contact rectifierswhich are useful for power applications in converting comparativelylarge amounts of alternating current power into direct current power.The meaning accorded the term single crystal, and equivalentterminology, as applied to the semiconductive bodies of the presentinvention, is a body of practically useful size in which thecrystallographic axes throughout the body have the same orientation.Although the individual grains of a compacted and sintered ceramic bodymay also be considered as single crystals, their individual small sizeprecludes their useful application as semiconductor bodies and as suchdo not fall within the above definition.

Although rectifying devices incorporating sintered and compacted ceramicbodies prepared according to the teachings of the aforementionedapplications have been found to exhibit good rectifier properties,numerous important advantages are realized by using single crystals asthe semiconductive material, as compared to rectifiers employingpolycrystalline semiconductive bodies. Included among the advantageswhich may be attributed to devices incorporating single crystals ofstrontium titanate, without limitation, are the absence of grainboundaries and the expected regions of weakness which may occur inrectifying barriers or layers; the assurance of attaining greaterhomogeneity in the chemical composition of the single crystal devicesand in the electrical properties of such devices; the facility for ahigher degree of manufacturing control in that the measured electricalproperties of a single crystal of semiconductive material yields abetter indication of the characters determining rectifier performance;and the provision of reater mobility or velocity for the electricalcurrent carriers resulting in a lower resistivity which makes availablea wider range of compositions having practically useful values ofresistivity for rectifiers yet capable of formation thereon of effective barrier or surface layers. The above factors, alone and incombination, make possible more efiicient rectifier manufacture, andassure better performance, reliability, and provide wider fields ofapplication.

The above brief description, as well as other objects, features, andadvantages of the present invention will be best appreciated byreference to the following detailed description of several presentlypreferred but illustrative embodiments, when taken in conjunction withthe accompanying drawings, wherein:

Fig. 1 is a perspective view, partly in section, showing an area contactor junction rectifier embodying features of the present invention;

Fig. 2 is a perspective view, partly in section, of the rectifierillustrated in Fig. 1 with the addition of a counter electrode;

Fig. 3 is a perspective view, partly in section, of a point contact typeof rectifier embodying features of the present invention; and,

Fig. 4 is a curve showing the relationship between current and voltageduring the formation of rectifying surfaces or junctions in theprocessing of area rectifiers in accordance with aspects of the presentinvention.

In accordance with the present invention, it has been found thatstrongly reduced single crystals of strontium titanate exhibit bothpoint contact and area rectification; and when appropriate electrodesare formed in contact with a single crystalline strontium titanate body,usually by techniques well known per se, such rectifiers exhibit usefulproperties both for the detection of radio frequency energy and for theconversion of power.

Referring now specifically to Fig. 1, there is shown an area contact orjunction rectifier which incorporates a single crystal of strontiumtitanate which has a thermally and anodically conditioned surface 10a, alead or manganese dioxide coating 12 formed in electrical contact withand coextensive with the conditioned surface 101:, a further conductivecoating 14 coextensive with the coating 12 and a base electrode18'coextensive with the surface of the single crystalline body 10 remotefrom the conditioned surface 10a. Optionally, and as shown in Fig. 2, afurther layer 16 may be superimposed upon the layer 12 to provide acounter electrode; the further layer 16 may be made of a soft metal,such as lead or cadmium.

A typical method for the preparation of single crystal area rectifierswill now be described in detail, it being expressly understood that themethod is subject to a latitude of variation as detailed in thementioned copending applications, except for the preparation of thesingle crystalline body 10. Rectifier bodies can be prepared from asingle crystalline boule of either the clear and unreduced strontiumtitanate or the darker reduced strontium titanate. The unreducedmaterial is fired in a dry reducing atmosphere to impart the desiredsemiconductive properties. Reduction may take place at a temperature ofbetween 1000 C. and 1400 C. for a time of between and minutes and in anatmosphere of between 10 to 100% purified hydrogen and the balance apurified inert gas, such as helium, with a total rate of flow between500 to 2000 cubic centimeters per minute. It is preferred to have all ofthe rectifier bodies cut from single crystals subjected to the abovecontrolled reduction. Accordingly if the bodies or plates 10 are cut outfrom the dark reduced boule, it is preferred to oxidize the reducedboule and then to re-reduce under the controlled conditions.

Following the reduction of the single-crystalline bodies 10, the severalelectrodes 12, 14, 16, and 18 are applied to the body, after suitabletreatment to provide the rectifying surface or barrier 10a. The surfacetreatment may consist of sand blasting the surface 1011 upon which therectifying barrier is formed. Following the sand blasting, which isoptional, the surface 10a may be thermally oxidized or conditioned. Thisis accomplished by firing the single crystals in an atmosphere furnace.Since only the one surface is to be formed with the rectifying barrier,it is preferable to place the units in the atmosphere furnace in amanner to expose only the surface 10a to the atmosphere. For bestresults, the side of the body removed from the surface 10a may beprotected during the thermal firing by any suitable protective coatingso that the surface Which is to make electrical connection to the baseelectrode 18 is subjected to a minimum degree of oxidation.

Following the thermal oxidation, the surface 10a is electrolyticallyconditioned. Such electrolytic conditioning and treatment involvesestablishing the body 10 as the anode in a plating bath and operatingthe bath at a current density sufiicient to anodically oxidize thesurface iiia. The parameters for anodic conditioning will be bestappreciated by considering the current voltage relationship illustratedin Fig. 4. As the current is slowly increased the voltage rises in alinear relationship. At the current value I a sharp and marked rise involtage occurs. For this polarizing current, the single crystalline bodyis anodically oxidized or conditioned. Although it is not necessary thatmetallic salts be present in the plating bath to achieve such anodicconditioning, it has been found that after conditioning or polarizationthe surface 10a will readily accept the required layer of manganese orlead dioxide making up the coating or layer 12. For example, plating maybe achieved following polarization by initially conditioning thesemiconductive body in a lead nitrate solution. It has been found thatthe value of the current of polarization I varies widely. Thereforeduring both the conditioning and plating, voltage control is moreappropriate for practicing the process. Following the polarization orconditioning, the voltage is cut back by an amount sutficient to operateat a fixed voltage V and a corresponding current I appropriate forplating from the bath containing the lead salts. A lead dioxide platedlayer of a few molecules thickness is sufiicient for the layer 12.

As pointed out in application Serial No. 365,987, rectificationproperties can be improved by applying a further metallic oxide coating14 to the plated layer 12. The layer i4 is preferably applied in theform of a paste which is prepared by intimately mixing the metal oxidepowder, for example lead dioxide or manganese dioxide, in a binder, suchas silicone cement. For further details reference may be made to thementioned copending application.

Finally the counter electrode 16 and the base electrode are applied, forexample by provision of solder, cadmium or similar metals.

For the point-contact rectifier illustrated in Fig. 3, the processing isessentially the same as for the area contact rectifier except that thearea layers 12, 14, 16 are replaced by a point 16' of a conductingmaterial, such as lead, tin, tungsten or lead dioxide.

Among the advantages which may be attributed to rectifiers embodying asemiconductive body of strontium titanate are the lower cost ofmaterials as compared to present devices incorporating germanium orsilicon and the assurance of greater temperature stability due to theinherent properties of the strontium titanate.

Experimental results indicate that single crystals of strontium titanateshow better rectification properties than polycrystalline materialsprepared by ceramic procedures. Grain boundaries may be expected to beregions of weakness in a recetifying barrier layer; such grainboundaries are absent in single crystals of strontium titanate. Further,it is possible to form barrier layers of more uniform properties due tothe greater homogeneity of the chemical composition and electricalproperties of single crystals. The measured electrical properties ofsingle crystals of strontium titanate give av better reflection of thefactors determining performance as a rectifier. This tends to make ahigher degree of manufacturing control possible. Still further themobility or velocity of the electron current carriers is greater forsingle crystals. As a result such single crystals have lower resistivitythan polycrystalline materials of the same composition. Thus there ismade available a wider range of compositions having practically usefulvalues of resistivity and on which at the same time sufiicientlyeffective barrier layers may be formed. Still further if any significantdifierence exists between the rectifying capabilities of various crystalfaces of a strontium titanate body such differences may be applied toadvantage by the correct orientation of the single crystal. As comparedto polycrystalline bodies of the same material, the rectifying surfacesconsist of a random mixture of various crystal faces.

Numerous other advantages of the present invention will occur to thoseskilled in the art and in some instances some features of the inventioncan be used without a corresponding use of other features. Accordinglythe appended claims should be construed broadly and as is consistentwith the disclosure.

What we claim is:

l. A rectifier comprising a single crystalline body of reduced strontiumtitanate having a thermally oxidized and anodically conditioned surface,and a metallic oxide layer formed on said surface.

2. In the manufacture of a titanate rectifier, the steps includingpreparing a single-crystalline body of reduced strontium titanate,introducing a surface of said body into an electrolytic bath containinga metallic salt which plates out as the metal dioxide, operating saidbath at a current density selected to anodically oxidize said surface ofsaid body and to condition said surface to receive said metal dioxide,and operating said bath at a reduced current density selected to plate alayer of said metal dioxide onto said surface.

3. In the manufacture of a rectifier, the steps of preparing asingle-crystalline body of a reduced strontium titanate, mechanicallyroughening and thermally oxidizing a surface of said body, anodicallyoxidizing said surface of said body to condition said surface to receivea metallic oxide layer, electrolytically depositing said metallic oxidelayer on said conditioned surface, and

coating the electrolytically deposited metallic layer with a furtherlayer of said metallic oxide.

4. In the manufacture of a rectifier, the steps including preparing arectifier body from a single crystalline boule of strontium titanate,firing said body in a reducing atmosphere at a temperature in the rangeof 1000' C. to 1400 C. to render said body semiconductive, thermallyoxidizing said surface of said body, anodically oxidizing said surfaceof said body, and electrolytically depositing a metallic oxide layeronto said surface.

References Cited in the file of this patent UNITED STATES PATENTS2,633,543 Howatt Mar. 31, 1953 2,711,496 Ruben June 21, 1955 2,711,497Ruben June 21, 1955 2,760,126 Kopelman Aug. 21, 1956 FOREIGN PATENTS497,474 Germany May 8, 1930

1. A RECTIFIER COMPRISING A SINGLE CRYSTALLINE BODY OF REDUCED STRONTIUMTITANATE HAVING A THERMALLY OXIDIZED AND ANODICALLY CONDITIONED SURFACE,AND A METALLIC OXIDE LAYER FORMED ON SAID SURFACE.